When an article to be molded is provided therein with an undercut portion spreading in the circumferential direction thereof, the punching-out of the undercut portion of the molded article has heretofore been performed forcibly so as to push and spread the undercut portion of the molded article to an extent enabling the punching-out utilizing elastic deformation of the material of the molded article. In performing the punching-out forcibly, however, the material of the molded article has been limited to a material capable of being elastically deformed with ease and problems of demolding failure and production of molded articles forming cracks therein has been entailed.
As a mechanism to solve the problems imposed by the forcible punching-out, there has been known a mechanism comprising plural cores lining radially as so-called collapsible cores and a center pin movably disposed at the center of the cores and having a structure in which the center pin is pushed in the inside of each core to expand a segment portion on the outer periphery of each core to a shape or size corresponding to that of an undercut portion, whereas the center pin is pulled out utilizing the opening or closing motion of a mold to move each core inward in the radial direction of the center pin, thereby reducing the core in diameter.
In addition, as an undercut processing mechanism different from the mechanism using the collapsible cores, there has also been known a mechanism comprising a first side plate provided on the upper side thereof with an undercut mold core portion and divided into two portions performing opening and closing in a vertical direction relative to the opening and closing direction of a mold and a second side plate similarly provided on the upper side thereof with an undercut mold core portion and divided into two portions performing opening and closing in the direction orthogonal to the opening and closing direction of the first side plate, thereby solving the drawbacks of the mechanism using the collapsible cores, which include growth in size and poor durability (refer, for example to JP 10-000660 A).